Knockdown Of Noxa Research Articles

Variations of Noxa Levels Caused by p38 MAPK Signaling Result In Heterogeneous Sensitivity to ABT-737 In CD40L-Stimulated CLL Cells

AbstractAbstract 1825Chronic lymphocytic leukemia (CLL) is characterized by relentless relapses. It is postulated that this might be due to microenvironment-induced resistance to cytotoxic agents. Previously we have shown that in vitro CD40 stimulation of peripheral blood derived from CLL patients can to a certain extent mimic the lymph node (LN) microenvironment and result in resistance to cytotoxic drugs. This is correlated with CD40-induced changes in apoptosis regulating Bcl-2 family members. The BH3 mimetic drug ABT-737 antagonizes the anti-apoptotic molecules Bcl-XL and Bcl-2, but not Mcl-1 or Bfl-1. At present it is unknown whether patient-specific variations in expression of apoptosis regulating proteins occur and to what extent they correlate with sensitivity or resistance to ABT-737.In the present investigation we assessed the variability in response to ABT-737 in CD40 stimulated CLL cells. Although in all tested CD40-stimulated CLL patients Mcl-1 and Bfl-1 levels increased, there was a heterogeneous response in cell death to ABT-737 when used as a single agent. Notably, time course analyses showed variations in Noxa levels upon CD40 triggering, which differed between CLL patients. High Noxa/Mcl-1 ratios correlated with sensitivity to ABT-737. This was confirmed by Noxa knockdown as well as Mcl-1 and Bfl-1 overexpression, which resulted in resistance to ABT-737. To determine the signaling pathways involved in Noxa regulation, CLL cells were stimulated with CD40L in the presence of various kinase inhibitors. We observed that Noxa levels were specifically inhibited by the p38 MAP kinase inhibitor SB203580. Preliminary data indicate that after CD40 stimulation, phosphorylation of p38 decreased over time, correlating with a decrease in Noxa levels. Importantly however, a minority of CD40 stimulated CLL samples were sensitive to ABT-737 and showed high levels of phosphorylated p38 and increasing Noxa levels over time.In conclusion, we demonstrate that following CD40 triggering of CLL cells the p38 MAPK signaling pathway regulates Noxa levels and that patient-specific variations in this pathway contribute to the responsiveness to ABT-737. These findings raise the question whether assessment of p38 MAPK activation status and Noxa levels in vivo could predict the response of CLL patients to ABT-737. Moreover our findings offer possible new targets for the treatment of CLL. Disclosures:No relevant conflicts of interest to declare.

A p38MAPK/HIF-1 Pathway Initiated by UVB Irradiation Is Required to Induce Noxa and Apoptosis of Human Keratinocytes

The signal transduction pathways leading to apoptosis of human keratinocytes responding to UVB irradiation are complex and not completely understood. Previously, we reported that in UVB-irradiated keratinocytes, p38(MAPK) instigates Bcl-2-associated X protein (Bax) activation and mitochondrial apoptosis. However, the molecular mechanism underlying the pro-apoptotic function of p38(MAPK) remained unclear. Here, we show that in UVB-treated human primary keratinocytes the activation of p38(MAPK) is necessary to upregulate Noxa, a BH3-only pro-apoptotic dominantly induced by UVB and required for apoptosis. Whereas p53-silencing was marginally cytoprotective and poorly affected Noxa expression, p38(MAPK) inhibition in p53-silenced keratinocytes or in p53(-/-) cells could still efficiently prevent Noxa induction and intrinsic apoptosis after UVB, indicating that p38(MAPK) signals mainly through p53-independent mechanisms. Furthermore, p38(MAPK) was required for the induction and activation of hypoxia-inducible factor 1 (HIF-1) in response to UVB, and HIF-1 knockdown reduced Noxa expression and apoptosis. In UVB-irradiated keratinocytes, Noxa targeted the anti-apoptotic myeloid cell leukemia sequence 1 (Mcl-1) for degradation, and small-interfering RNA (siRNA)-mediated knockdown of Noxa or p38(MAPK) inhibition restored levels of Mcl-1 and abolished apoptosis. Thus, the pro-apoptotic mechanisms orchestrated by p38(MAPK) in human keratinocytes in response to UVB involve an HIF-1/Noxa axis, which prompts the downregulation of anti-apoptotic Mcl-1, thereby favoring Bax-mediated mitochondrial apoptosis of UVB-damaged keratinocytes.

Noxa mediates hepatic stellate cell apoptosis by proteasome inhibition

Induction of hepatic stellate cell (HSC) apoptosis is a viable therapeutic strategy to reduce liver fibrogenesis. Although BH3-only proteins of the Bcl-2 family trigger pro-apoptotic pathways, the BH3-only proteins mediating HSC apoptosis have not been well defined. Our aim, using proteasome inhibition as a model to induce HSC apoptosis, was to examine the BH3-only proteins contributing to cell death of this key liver cell subtype. Apoptosis was induced by treating LX-2 cells, an immortalized human hepatic stellate cell line, and primary rat stellate cells with the proteasome inhibitor MG-132. Treatment with proteasome inhibitors increased expression of Noxa both at the mRNA (16-fold) and protein (22-fold) levels indicating that both transcriptional and post-translational mechanisms contributed to the increase in cellular Noxa levels. Knockdown of Noxa by siRNA significantly attenuated cell death, mechanistically implicating Noxa as a key apoptotic mediator of proteasome inhibitor-induced cell death. Given the pivotal role for the anti-apoptotic Bcl-2 protein A1 in activated HSC survival, we determined if Noxa bound to this survival protein. Noxa was shown to physically bind the anti-apoptotic Bcl-2 protein A1 by co-immunoprecipitation. Noxa contributes to proteasome inhibitor-induced apoptosis of stellate cells likely by binding A1. Strategies to therapeutically increase Noxa expression may be useful for inducing HSC apoptosis.

PS-341 and Histone Deacetylase Inhibitor Synergistically Induce Apoptosis in Head and Neck Squamous Cell Carcinoma Cells

Proteasome inhibitor PS-341 (also known as bortezomib) and histone deacetylase (HDAC) inhibitors have emerged as novel therapeutic agents for a variety of malignancies. In this study, we examined whether PS-341 and the HDAC inhibitor trichostatin A (TSA) induced apoptosis in head and neck squamous cell carcinoma (HNSCC), a common and lethal malignancy. We found that, although TSA treatment alone did not induce apoptosis in HNSCC cells, it significantly enhanced PS-341-induced apoptosis in HNSCC cells in vitro. Consistently, TSA significantly improved PS-341-mediated inhibition of HNSCC tumor growth in nude mice. Mechanistically, we found that TSA increased PS-341-induced Noxa expression and caspase activation in HNSCC cells. The knockdown of Noxa significantly reduced apoptosis induced by cotreatment of PS-341 and TSA. Taken together, our results provide new insight into the mechanisms of synergistic antitumor activity of the PS-341 and HDAC inhibitor regimen, offering a new therapeutic strategy for HNSCC patients.

Abstract 3493: ON 01910. Na, a clinical trial stage multi-kinase inhibitor, induces apoptosis in chronic lymphocytic leukemia (CLL) cells through inhibition of PI3K/AKT and activation of the JNK pathway resulting in NOXA and BIM upregulation

Abstract Chronic Lymphocytic leukemia (CLL), the most common leukemia in Western countries, is a clinically heterogeneous disease characterized by the accumulation of mature B lymphocytes, where disease progression is driven by survival and proliferation signals provided by the tumor microenvironment. ON 01910. Na (Onconova Therapeutics) belongs to the family of styryl benzyl sulfones, a novel family of non-ATP competitive kinase inhibitors. ON 01910. Na is under clinical development in hematologic and solid tumors. Here we show that ON 01910. Na induced apoptosis in CLL samples (n=28) in vitro, without showing significant toxicity against T-cells or normal B-cells. Twenty four CLL samples were highly sensitive to ON 01910. Na (mean IC50=0.89 μM) a concentration readily achieved in phase I clinical trials, three showed moderate sensitivity (mean IC50=4.26 μM) and one was resistant (IC50 >8 μM). There were no significant differences in ON01910. Na cytotoxicity against CLL cells expressing mutated (mean IC50=1.33 μM) or unmutated (mean IC50 =1.22μM) IgVH sequences. ON 01910. Na was similarly effective against tumor cells carrying 17p or 11q deletions. ON 01910. Na activated Bax and Bak, leading to mitochondrial depolarization. To delineate the upstream pathways underlying ON 01910. Na induced apoptosis, we performed Gene Expression Profiling (GEP) in CLL cells treated in vitro for 4 and 10 hours with ON 01910. Na. Gene Set Enrichment Analysis (GSEA) identified gene sets indicating BCR and PI3K inhibition (FDR<0.2). In keeping with PI3K inhibition, we found that ON 19010. Na inhibited phosphorylation of Akt and Foxo3a, and FOXO3 target genes, including Bim, were upregulated. In addition, GSEA identified induction of AP-1 gene sets (FDR<0.01) suggesting the generation of reactive oxygen species (ROS) by ON 01910. Na. Consistently, we found a rapid increase in ROS in cells treated with ON 01910. Na and ROS scavengers protected cells from its cytotoxic effect. ROS upregulated the proapoptotic BH3-only protein Noxa, and knockdown of Noxa by shRNA decreased the sensitivity to the drug by 60%. Our results identify ON 01910. Na as a promising agent in the treatment of CLL with an interesting dual mechanism of action: inhibition of the PI3K/AKT survival pathway with Foxo mediated upregulation of BIM and the induction of oxidative stress resulting in Noxa upregulation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3493.

Abstract 1030: Arsenic trioxide plus ethacrynic acid-butyl ester is synergistic to induce apoptosis in myeloid leukemia cells through upregulation of NOXA

Abstract Arsenic trioxide (As2O3) induces complete remission in acute promyelocytic leukemia (APL) patients. However, other types of myeloid leukemia patients are less responsive to As2O3 treatment. APL t(15;17) NB4 cells are more sensitive to As2O3 apoptosis induction than other types of myeloid leukemia cells such as HL-60 and K562 cells. Previously we reported that As2O3 induces apoptosis in NB4 cells through a reactive oxygen species (ROS)-mediated pathway which is attenuated by the expression of glutathione-s-transferase π (GSTπ). NB4 cells contain lower levels of GSTπ than other types of myeloid leukemia cells. Therefore, inhibition of GSTπ should enhance As2O3 induction of apoptosis in other types of myeloid leukemia cells. We investigated the combined effect of As2O3 with ethacrynic acid (EA) and a more potent non-diuretic EA analogue ethacrynic acid-butyl ester (EABE), known GSTπ inhibitors, to induce apoptosis in HL-60 and K562 cells. As2O3 at clinical achievable concentrations (1-2 µM) did not induce apoptosis in HL-60 cells nor in K562 cells. EA at a concentration lower than 60 μM did not induce apoptosis in both cell lines either. As2O3 at 2 μM plus EA at 60 μM are synergistic to induce apoptosis in both HL-60 and K562 cells as measured by Annexin V staining, PARP cleavage and caspase activation. More intriguing, EABE at 1 μM (60 fold lower than EA), which inhibits GSTπ activity, exhibited synergistic induction with As2O3 in K562 cells. Combined index of As2O3 with EABE to induce apoptosis in K562 cells calculated by CompuSyn software was less than 1.0 which indicated synergism. Apoptosis induction by the combination treatment of As2O3 with EABE correlated with the upregulation of the Bcl-2 homology 3 (BH3)-only pro-apoptotic protein NOXA. Knock-down of NOXA using siRNA blocked apoptosis induction of As2O3 plus EABE, while, knock-down of Mcl-1 enhanced the apoptosis induction of As2O3 plus EABE. Immunoprecipiation with Mcl-1 antibody revealed that NOXA bond to Mcl-1 in the K562 cells treated with As2O3 plus EABE. Inhibition of ROS production did not inhibit the upregulation of NOXA nor the apoptosis induction of As2O3 plus EABE treatment. Our data suggest that As2O3 plus EABE induce apoptosis through a novel mechanism which induces NOXA and sequesters Mcl-1 leading to the activation the intrinsic apoptotic pathway. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1030.

Abstract 667: Mechanisms underlying synergistic interactions between the CDK inhibitor flavopiridol (Alvocidib) and the BH3 mimetic GX15-070 (Obatoclax) in human multiple myeloma cells

Abstract To establish a mechanistic basis for a strategy combining CDK inhibitors and Bcl-2 antagonists, in vitro and in vivo interactions between flavopiridol (FP) and GX15-070 (GX) were investigated in human multiple myeloma (MM) cells. The activity of this regimen was also examined in primary CD138+ MM cells and in a murine MM xenograft model. Co-administration of sub-toxic concentrations of FP interacted synergistically with GX to induce apoptosis in U266, RPMI8226, H929, OPM-2, and MM.1S, as well as IL-6-dependent ANBL6 and KAS 6/1 MM cells. Combination treatment also effectively killed MM cells in the presence of stromal cells or growth factors. The regimen was active against multiple drug-resistant MM cell lines, including bortezomib/Velcade- (U266/VR and OPM-2/VR), Revlimid- (8226/R10R), dexamethasone- (MM.1R), melphalan- (8226/LR5), and doxorubicin-resistant (8226/Dox40) MM cells. FP also sharply promoted GX lethality in primary CD138+ MM cells, while sparing normal CD138- bone marrow cells. GX induced clear declines in Mcl-1 and Bcl-xL levels which were potentiated by FP, whereas Bcl-2 expression did not change. Significantly, FP/GX overcame cytoprotective effects mediated by ectopic Mcl-1, Bcl-xL, or Bcl-2 expression. Notably, BH3-only protein expression profiling revealed multiple changes: a) FP, in the presence or absence of GX, triggered a clear increase in Bim expression (both BimEL and BimL isoforms); b) FP/GX co-treatment sharply induced Bik (∼35kDa species) and Noxa (∼27 kDa species),; c) co-exposure (24hr) to FP and GX induced a clear reduction in Puma expression; and c) no changes in the expression of other BH3-only proteins occurred. These findings were confirmed in various MM cell lines, including multiple drug-resistant cells, and were associated with Bak/Bax activation, Bax translocation, cytochrome C release, and caspase activation. Importantly, Bim, Bik, or Noxa knock-down by stable transfection with shRNA dramatically diminished FP/GX lethality. Lastly, co-administration of FP and GX significantly suppressed tumor growth, reflected by diminished tumor size and bioluminescence intensity, in athymic nude mice inoculated with 8226 cells stably expressing luciferase. These events were associated with pronounced apoptosis, manifested by caspase 3 activation and PARP degradation. Western blot analysis also revealed down-regulation of Mcl-1, as well as up-regulation of Bik and Noxa in tumor tissues following FP/GX co-administration. Collectively, these findings provide a mechanistic framework for a strategy combining CDK inhibitors with Bcl-2 antagonists in which down-regulation of anti-apoptotic proteins and induction of BH3-only proteins cooperate to trigger synergistic killing of MM cells, including those resistant to either conventional or novel anti-MM agents, both in vitro and in vivo. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 667.

Abstract 1200: Oncogenic KRAS sensitizes colorectal tumor cells to chemotherapy via p53-dependent induction of Noxa

Abstract Oxaliplatin and 5-fluorouracil currently form the backbone of conservative treatment in patients with metastatic colorectal cancer. Tumor responses to these agents are highly variable, but the underlying mechanisms are poorly understood. Our previous results have indicated that the presence of oncogenic KRAS in colorectal tumor cells sensitizes these cells to chemotherapy. In the present manuscript we show that the sensitization of colorectal tumor cells to oxaliplatin and 5-FU by oncogenic KRAS is critically dependent on the presence of wild type p53. KRAS status did not influence p53 stabilization, p21 induction, or cell cycle arrest. A screen for induction of apoptosis regulators revealed that KRAS selectively promoted expression of the pro-apoptotic p53 target gene Noxa. Suppression of Noxa by RNA interference strongly reduced KRAS/p53-dependent apoptosis following exposure to chemotherapy, both in vitro and in tumor xenografts in mice. Noxa knockdown did not affect p53-dependent p21 induction or cell cycle arrest. Our results identify oncogenic KRAS as an important determinant of the cellular response to p53 activation following exposure to oxaliplatin or 5-FU, by facilitating apoptosis induction through Noxa. Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1200.

The Anti-apoptotic Protein BCL2L1/Bcl-xL Is Neutralized by Pro-apoptotic PMAIP1/Noxa in Neuroblastoma, Thereby Determining Bortezomib Sensitivity Independent of Prosurvival MCL1 Expression

Neuroblastoma is the most frequent extracranial solid tumor in children. Here, we report that the proteasome inhibitor bortezomib (PS-341, Velcade) activated the pro-apoptotic BH3-only proteins PMAIP1/Noxa and BBC3/Puma and induced accumulation of anti-apoptotic MCL1 as well as repression of anti-apoptotic BCL2L1/Bcl-xL. Retroviral expression of Bcl-xL, but not of MCL1, prevented apoptosis by bortezomib. Gene knockdown of Noxa by shRNA technology significantly reduced apoptosis, whereas Puma knockdown did not affect cell death kinetics. Immunoprecipitation revealed that endogenous Noxa associated with both, Bcl-xL and MCL1, suggesting that in neuronal cells Noxa can neutralize Bcl-xL, explaining the pronounced protective effect of Bcl-xL. Tetracycline-regulated Noxa expression did not trigger cell death per se but sensitized to bortezomib treatment in a dose-dependent manner. This implies that the induction of Noxa is necessary but not sufficient for bortezomib-induced apoptosis. We conclude that MCL1 steady-state expression levels do not affect sensitivity to proteasome-inhibitor treatment in neuronal tumor cells, and that both the repression of Bcl-xL and the activation of Noxa are necessary for bortezomib-induced cell death.

Aspirin induces apoptosis in human leukemia cells independently of NF-κB and MAPKs through alteration of the Mcl-1/Noxa balance

Aspirin and other non-steroidal anti-inflammatory drugs induce apoptosis in most cell types. In this study we examined the mechanism of aspirin-induced apoptosis in human leukemia cells. We analyzed the role of nuclear factor-kappaB (NF-kappaB) and mitogen-activated protein kinases (MAPKs) pathways. Furthermore, we studied the changes induced by aspirin in some genes involved in the control of apoptosis at mRNA level, by performing reverse transcriptase multiplex ligation-dependent probe amplification (RT-MLPA), and at protein level by Western blot. Our results show that aspirin induced apoptosis in leukemia Jurkat T cells independently of NF-kappaB. Although aspirin induced p38 MAPK and c-Jun N-terminal kinase activation, selective inhibitors of these kinases did not inhibit aspirin-induced apoptosis. We studied the regulation of Bcl-2 family members in aspirin-induced apoptosis. Aspirin increased the mRNA levels of some pro-apoptotic members, such as BIM, NOXA, BMF or PUMA, but their protein levels did not change. In contrast, aspirin decreased the protein levels of Mcl-1. Interestingly, in the presence of aspirin the protein levels of Noxa remained high. This alteration of the Mcl-1/Noxa balance was also found in other leukemia cell lines and primary chronic lymphocytic leukemia cells (CLL). Furthermore, in CLL cells aspirin induced an increase in the protein levels of Noxa. Knockdown of Noxa or Puma significantly attenuated aspirin-induced apoptosis. These results indicate that aspirin induces apoptosis through alteration of the Mcl-1/ Noxa balance.

Aspirin Induces Apoptosis in Human Leukemia Cells Independently of NF-κB and MAPKs through Alteration of the Mcl-1/Noxa Balance.

Abstract 4825Aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs) induce apoptosis in most cell types. We examined the mechanism of aspirin-induced apoptosis in human leukemia cells. Our results show that aspirin induced apoptosis in leukemia Jurkat T cells independently of NF-κB. Although aspirin induced p38 MAPK and c-Jun N-terminal kinase (JNK) activation, selective inhibitors of these kinases did not inhibit aspirin-induced apoptosis. We studied the regulation of Bcl-2 family members in aspirin-induced apoptosis. The mRNA levels of some pro-apoptotic members, such as BIM, NOXA, BMF or PUMA, were induced by aspirin. However, none of these pro-apoptotic proteins increased and the levels of Mcl-1 protein were reduced. Interestingly, in the presence of aspirin the protein levels of Noxa remained high. This alteration of the Mcl-1/Noxa balance was also found in other leukemia cell lines and primary chronic lymphocytic leukemia cells (CLL). Furthermore, in CLL cells aspirin induced an increase in the protein levels of Noxa. Knockdown of Noxa or Puma significantly attenuated aspirin-induced apoptosis. These results indicate that aspirin induces apoptosis through alteration of the Mcl-1/Noxa balance. DisclosuresNo relevant conflicts of interest to declare.

Targeting CDK9 Dramatically Potentiates ABT-737-Induced Apoptosis in Human Multiple Myeloma Cells through a Bim-Dependent Mechanism.

Abstract 297 Background:High expression of the anti-apoptotic Bcl-2 family protein Mcl-1 is a critical survival mechanism in human multiple myeloma (MM) cells, and a key barrier to apoptosis induced by the Bcl-2/-xL antagonist ABT-737. De novo expression of the short-lived protein Mcl-1 is controlled transcriptionally by CDK9, a catalytic subunit of the PTEF-b transcription elongation complex (CDK9/cyclin T) that phosphorylates the C-terminal domain (CTD) of RNA polymerase II (RNA pol II) to promote elongation of nascent transcripts. Thus, approaches targeting CDK9 may enhance ABT-737 efficacy in MM cells. Methods:To test this hypothesis, we employed the pan-CDK inhibitor roscovitine and a highly-selective CDK9 inhibitor (CDK9i) in drug-naïve and drug-resistant MM cell lines, as well primary plasma cells. Results:Co-administration of non- or sub-toxic concentrations of roscovitine or CDK9i interacted in a highly synergistic manner with ABT-737 (e.g., 150-300 nM) to induce apoptosis in U266, RPMI 8226, H929, OPM-2, and MM.1S MM cells, as well as IL-6-depedent ANBL-6 and KAS 6/1 cells. In each case, Combination Index values were significantly less than 1.0. These regimens were fully active in various drug-resistant MM cells, including U266/VR and OPM-2/VR bortezomib-resistant cells, RPMI 8226/RR lenalidomide-resistant cells, MM.1R dexamethasone-resistant cells, RPMI 8226/LR5 melphalan-resistant cells, and RPMI 8226/Dox40 doxorubicin-resistant cells. They also effectively killed MM cells in the presence of stromal cells or growth factors (i.e., IL-6, IGF-1, BAFF, and APRIL). Co-administration of roscovitine significantly enhanced ABT-737 lethality in primary CD138+ MM cells, while largely sparing CD138− bone marrow cells. Notably, the majority of MM cell lines exhibited high expression of cyclin T1 rather than cyclin T2a/b, in association with constitutively-activated CDK9, manifested by high levels of phosphorylation of CDK9 and RNA pol II CTD at Ser2 and 5. Exposure of MM cells to either roscovitine or CDK9i strikingly reduced RNA pol II CTD phosphorylation at both sites (particularly Ser5), but did not affect CDK9 and cyclin T1 levels. Whereas treatment with ABT-737 alone led to a modest but clear increase in Mcl-1 expression, roscovitine or CDK9i completely blocked this event and substantially down-regulated basal Mcl-1 expression. Notably, similar phenomena occurred in U266/VR cells, which displayed higher basal levels of Mcl-1, an important candidate resistance mechanism, compared to parental cells. Moreover, knock down of Mcl-1 by stable transfection with shRNA dramatically increased U266 sensitivity to ABT-737. Co-treatment of MM cells with ABT-737 and roscovitine or CDK9i did not affect expression of other anti-apoptotic Bcl-2 family members such as Bcl-2 and Bcl-xL, but induced a Bcl-2 cleavage fragment. Expression profiling of BH3-only proteins in MM cells co-exposed to roscovitine and ABT-737 revealed a modest or a marked increase in expression of Bim (both EL and L isoforms) and Noxa (particularly a ∼27 kDa species), respectively, accompanied by a clear decrease in Puma levels. No changes in expression of other BH-3-only proteins (e.g., Bid, Bad, Bik, BMF, Hrk) were observed. Interestingly, knock down of Bim by stable transfection with shRNA dramatically diminished roscovitine/ABT-737 lethality, whereas Noxa knock-down by shRNA only partially protected cells from this regimen. In contrast, both shRNA approaches substantially protected U266 cells from bortezomib lethality. Conclusions:Collectively, these findings provide a mechanistic framework for targeting transcription by CDK9 inhibitors to diminish ABT-737-mediated Mcl-1 upregulation and to downregulate basal Mcl-1 levels, resulting in highly synergistic killing of MM cells, including those resistant to either conventional or novel anti-MM agents. They also suggest that the BH3-only protein Bim may be required for apoptosis triggered by this strategy, while up-regulation of Noxa may amplify MM cell killing in this setting. Disclosures:No relevant conflicts of interest to declare.

Human Melanoma Cells under Endoplasmic Reticulum Stress Are More Susceptible to Apoptosis Induced by the BH3 Mimetic Obatoclax

Past studies have shown that melanoma cells have largely adapted to endoplasmic reticulum (ER) stress, and this is associated with up-regulation of the antiapoptotic proteins Bcl-2 and Mcl-1. In this report, we show that the BH3 mimetic obatoclax potently overcomes resistance of melanoma cells to apoptosis induced by ER stress. Obatoclax, as a single agent at nanomolar concentrations, was relatively ineffective in the induction of apoptosis in melanoma cells, but treatment with obatoclax at these concentrations in combination with the ER stress inducer tunicamycin (TM) or thapsigargin markedly enhanced apoptotic cell death. This was primarily because of the inhibition of Mcl-1 by obatoclax, in that cotreatment with TM and another BH3 mimetic ABT737, which does not antagonize Mcl-1, caused only minimal increases in apoptosis. Moreover, overexpression of Mcl-1 inhibited apoptosis to greater degrees than overexpression of Bcl-2. In addition to direct inhibition of Mcl-1 by obatoclax, the combination of obatoclax and TM caused strong up-regulation of the BH3-only protein Noxa. Small RNA interference knockdown of Noxa partially inhibited apoptosis induced by cotreatment with obatoclax and TM. Similarly, knockdown of Bak also blocked induction of apoptosis by the compounds. The Mcl-1/Bak interaction seemed to be disrupted more efficiently in melanoma cells cotreated with obatoclax and TM. Taken together, these results identify obatoclax as a potent agent that overcomes resistance of melanoma cells to ER stress-induced apoptosis and seem to have important implications in the use of BH3 mimetics in the treatment of melanoma.

Effect of (-)-gossypol (AT-101) on transcriptional regulation of Noxa and Puma and on Mcl-1-mediated cancer cell resistance to apoptosis

e14611 Background: AT-101 is being evaluated in Phase II clinical trials for the treatment of human cancers and demonstrates anticancer activity as a single agent and in combination with docetaxel. Despite extensive investigations, the mechanism of action for its antitumor activity is not completely understood. In this study, we have performed detailed investigations using a panel of human cancer cell lines on the mechanism of action for AT-101. Methods: A series of human breast and prostate cancer cell lines were utilized to evaluate the antitumor activity of AT-101. Cell growth inhibition, cell viability and apoptosis were determined by WST assay, trypan blue assay and flow cytometry, respectively. Gene specific siRNAs were used to knockdown target gene expression. Quantitative RT- PCR was performed to detect the mRNA levels of Puma and Noxa. Co-immunoprecipitation was carried out to detect the association between Mcl-1 and Noxa or Puma. Results: AT-101 induced apoptosis in both Bax/Bak-dependent and -independent manners in a variety of human cancer cell lines concomitant with increased expression of Noxa and Puma in a p53-independent manner. The up-regulation of Noxa was not observed in normal primary breast epithelial cells or normal-like human breast epithelial MCF-12F cells. Transcription inhibitor actinomycin D blocked AT-101-induced up-regulation of Puma and Noxa. Knockdown of Noxa or Puma attenuated AT-101-induced apoptosis. Furthermore, AT-101 effectively overcame Mcl-1-mediated cancer cell resistance to apoptosis. Co-immunoprecipitation studies demonstrated that Mcl-1 specifically associated with Noxa but not Puma. Conclusions: Our findings suggest that transcriptional up-regulation of pro-apoptotic Noxa and Puma contributes to the antitumor activity of AT-101, which plays a dominant role in antagonizing Mcl-1 and overcoming Mcl-1-mediated resistance to apoptosis of cancer cells. AT-101 functions as an antagonist of Bcl-2, Bcl-xL and Mcl-1 through direct binding to these proteins, as well as through upregulation of Noxa and Puma. [Table: see text]

Induction of Noxa Sensitizes Human Colorectal Cancer Cells Expressing Mcl-1 to the Small-Molecule Bcl-2/Bcl-xL Inhibitor, ABT-737

The intrinsic drug resistance of colorectal cancers is related in part to overexpression of prosurvival Bcl-2 family proteins. We determined the effects of ABT-737, a small-molecule inhibitor of Bcl-2/Bcl-xL but not Mcl-1, on apoptosis induction alone and in combination with CPT-11 and explored mechanisms underlying their cooperativity. Human colorectal carcinoma cell lines (HCT116 wild-type and Bax(-/-), HT-29, and RKO) were incubated with ABT-737 alone and combined with CPT-11 or bortezomib, and cell viability, caspase cleavage, and Annexin V labeling were measured. In drug-treated cell lines, protein-protein interactions were analyzed by immunoprecipitation. Lentiviral short hairpin RNA was used to knockdown Noxa expression. ABT-737 induced apoptosis in a dose-dependent manner and its coadministration with the topoisomerase I inhibitor, CPT-11, resulted in a synergistic cytotoxic effect. Apoptosis induction by the drug combination was associated with enhanced caspase-8, caspase-9, and caspase-3 activation and poly(ADP-ribose) polymerase cleavage that were completely abrogated in Bax knockout cells. ABT-737 unsequestered the BH3-only protein Bim from its complex with Bcl-xL or Bcl-2 and disrupted the interaction of Bcl-xL with Bak. CPT-11 treatment up-regulated Noxa expression, as did bortezomib, and enhanced Noxa/Mcl-1 complexes. CPT-11 also disrupted the Mcl-1/Bak interaction. Knockdown of Noxa using short hairpin RNA lentiviral constructs was shown to significantly attenuate the cytotoxic effect of CPT-11 or bortezomib combined with ABT-737 and inhibited caspase-3 cleavage. Induction of Noxa by CPT-11 or bortezomib can sensitize colorectal cancer cells expressing Mcl-1 to ABT-737. Up-regulation of Noxa may therefore represent an important strategy to enhance the therapeutic efficacy of ABT-737 against colorectal cancer and other solid tumors.

Inhibition of apoptosis facilitates necrosis induced by cisplatin in gastric cancer cells

Although cisplatin has been shown to induce both apoptosis and necrosis in cancer cells, the potential interconnections between these modes of cell death induced by the drug remain unknown. We studied this phenomenon in gastric cancer cell lines and identified one cell line (SGC-7901) that underwent apoptosis, and another cell line (BGC-823) that primarily underwent nonapoptotic cell death, in response to cisplatin. Apoptosis in cisplatin-treated SGC-7901 cells seemed to be caspase dependent and was mediated, at least in part, by the BH3-only protein, Noxa. This was evidenced by the rapid upregulation of Noxa and inhibition of apoptosis by small interfering RNA knockdown of Noxa. Nonapoptotic cell death induced by cisplatin in BGC-823 cells was characterized by lack of DNA fragmentation, delayed externalization of phosphatidylserine, caspase independence, plasma membrane disruption, and intracellular vacuole formation, indicative of necrosis. Surprisingly, blockage of apoptosis induction by a general caspase inhibitor or by Noxa small interfering RNA in SGC-7901 failed to protect against cisplatin-induced cell death. Under such conditions, SGC-7901 cells displayed cellular features associated with necrosis. Cisplatin-induced apoptosis, thus, seems to precede necrosis when the apoptotic machinery is operative. When the apoptosis program is defective, necrotic cell death takes place as an alternative pathway leading to cell demise. Induction of different modes of cell death that are interrelated in the same cells by cisplatin has the potential to be exploited in formulating new adjuvant cancer therapies.

C-Myc to Melanoma Cells: Drop Dead?

Because the proteasome regulates the cell cycle and apoptosis, proteasome inhibitor-mediated tumor cell death is not that surprising. What is puzzling, however, given the vital role of the proteasome in protein turnover, is the paucity of side effects on normal cells. What makes protease inhibitors so tumor cell-selective? One such compound, bortezomib, increases the abundance, in cancer cells but not in normal cells, of the mRNA and protein of the proapoptotic factor, NOXA. Nikiforov et al . investigated the mechanism by which bortezomib causes cell death in melanoma cell lines. Western blotting analysis showed that the bortezomib-dependent increase in NOXA abundance in melanoma cells was blocked by the transcriptional inhibitor actinomysin D. Bortezomib still increased the abundance of NOXA in cell lines that were treated with short interfering RNAs specific for those transcription factors known to target NOXA : p53, hypoxia-inducible factor-1α (HIF-1α), and E2F-1. Computational analysis of the sequences surrounding the NOXA promoter revealed the presence of four putative binding sites for the oncoprotein c-Myc. Chromatin immunoprecipitation assays confirmed the binding of c-Myc to the NOXA promoter, whose abundance was increased, compared with control cells, after treatment with bortezomib. Knockdown of c-Myc in melanoma cells blocked the bortezomib-dependent increase in NOXA abundance, whereas overexpression of c-Myc in normal melanocytes increased the abundance of NOXA mRNA, which was further enhanced by bortezomib treatment. Overexpression of c-Myc in melanoma cells sensitized them to the effects of bortezomib, whereas knockdown of NOXA inhibited bortezomib-induced cell death. Together, these data suggest the unusual involvement of c-Myc in promoting apoptosis and provide a possible mechanism for the tumor cell-specific effect of proteasome inhibitors. M. A. Nikiforov, M. Riblett, W.-H. Tang, V. Gratchouck, D. Zhuang, Y. Fernandez, M. Verhaegen, S. Varambally, A. M. Chinnaiyan, A. J. Jakubowiak, M. S. Soengas, Tumor cell-selective regulation of NOXA by c-MYC in response to proteasome inhibition. Proc. Natl. Acad. Sci. U.S.A. 104 , 19488-19493 (2007). [Abstract] [Full Text]

Proteasome Inhibitor PS-341 Induces Apoptosis in Cisplatin-resistant Squamous Cell Carcinoma Cells by Induction of Noxa

Proteasome Inhibitor PS-341 Induces Apoptosis in Cisplatin-resistant Squamous Cell Carcinoma Cells by Induction of Noxa

Proteasome Inhibitor PS-341 Induces Apoptosis in Cisplatin-resistant Squamous Cell Carcinoma Cells by Induction of Noxa

Cisplatin is one of the most common DNA-damaging agents used for treating patients with solid tumors such as squamous cell carcinoma (SCC). Unfortunately, significant levels of resistance in SCC cells emerge rapidly following cisplatin treatment. Here we report that the proteasome inhibitor PS-341, the representative of a new class of chemotherapeutic drugs, was capable of inducing apoptosis in cisplatin-resistant SCC cells via the endoplasmic reticulum stress. PS-341 stimulated the phosphorylation of PERK and the unfolded protein response, resulting in the induction of the transcription factor ATF-4. Importantly, the Bcl-2 homology domain 3-only (BH3-only) protein Noxa was found to be strongly induced in cisplatin-resistant SCC cells by PS-341 but not by cisplatin. The knock-down of Noxa using small interference RNA significantly abolished PS-341-mediated apoptosis in SCC cells. Using eIF2alpha mutant mouse embryonic fibroblasts, we found that functional eIF2alpha played an essential role in PS-341-induced Noxa expression. Taken together, our novel findings reveal a direct link between PS-341-induced endoplasmic reticulum stress and the mitochondria-dependent apoptotic pathway and suggest that PS-341 may be utilized for overcoming cisplatin-resistance in human SCC.

FKHRL1-mediated expression of Noxa and Bim induces apoptosis via the mitochondria in neuroblastoma cells

Protein kinase-B (PKB) and its target, the forkhead transcription factor like 1 (FKHRL1)/FoxO3a, have been suggested as regulators of neurotrophin-mediated cell survival in neuronal cells. We analyzed human neuroblastoma cells and found that FKHRL1 was phosphorylated, suggesting its inactivation. To study FKHRL1 function, we infected SH-EP and NB15 cells with a 4OH-tamoxifen-regulated FKHRL1(A3)ER(tm) transgene. Activation of FKHRL1 promoted cytochrome-c release and caspase-dependent apoptosis. FKHRL1 induced TRAIL and the BH3-only proteins Noxa and Bim, implicating both extrinsic and intrinsic death pathways. However, expression of dnFADD did not inhibit FKHRL1-induced cell death, whereas Bcl2 protected against apoptosis. This excluded the death-receptor pathway and suggested that cell death decision is regulated by Bcl2-rheostat. Importantly, RNAi knockdown of Noxa or Bim decreased apoptosis, indicating that Noxa and Bim cooperate to mediate FKHRL1-induced cell death. We conclude that Noxa and Bim establish a connection between FKHRL1 and mitochondria, and that both BH3-only proteins are critically involved in FKHRL1-induced apoptosis in neuroblastoma.